There's another perennial topic that's raised its head on one of the forums: multi-classing. But James Maliszewski has also opened a discussion about classes, sub-classes, and split classes on Grognardia, so I might as well combine all these topics.
I've mentioned before, when I was designing my own substitute class system. that I've come to think of a class as an archetype expressed as an answer to the question "How does this character solve problems?" The Fighter solves problems by fighting or direct physical force, the Magic-User by magic, the Cleric by faith, the Thief by subtlety and stealth. I have problems with the Thief as usually presented and the other classes in AD&D because they violate this rule, mainly because they interpret class more like a profession. I think things like skill in woodland tracking should be a separate ability potentially available to any class, based on character concept, rather than a class benefit.
A sub-class is one of the fundamental classes re-purposed. There are two ways to go about this: the wrong way and the right way. The wrong way would be the way I just described: taking one of the base classes and just piling on goodies that fit into a specific professional concept. The right way, in my opinion, is to do what I attempted to do with variant classes: substitute a variant ability for one of the existing class abilities. Like, for example, swapping Command Animal for Turn Undead and using a druid spell list to create a druid from the Cleric class, instead of using the druid class from Eldritch Wizardry.
A split class (or the prestige classes from 3e) is more like a base class that turns into a sub-class after you reach a specific level. I'm not sure that I really feel the need for split classes. If I had to, maybe I'd implement split classes as variant classes that characters could change to, using the standard dual classing rules.
Now, as for multi-classing, I think it got a little out of hand in later editions. Originally, only Elves could multi-class, and in only one combination: Fighter/Magic-User. To me, it represents the fact that elves are inherently magical, such that they can cast spells even if they are as skilled as fighters. When the thief class was added, this elven archetype was broken, because now elves could opt to be Fighter/Magic-User/Thieves or just plain Thieves. I would change this to either Fighter or Thief, but always Magic-User, and no triple multi-class.
For other races, I would rather not have multi-classing except through dual classing: in other words, a dwarf Fighter could switch to Thief if the dwarf has a Dex of 16+. And I'd add a house-rule for dual classing: dual class characters can switch back to the original class if they have pre-requisites of 16+ for both classes. This still keeps Elves special: they can switch back and forth between Fighter (or Thief) and Magic-User at any time, no matter what ability scores they have. To be a triple multi-class, they would need to meet the same minimum prerequisite requirement as other dual class characters.
What I really wanted to talk about in terms of multi-classing was the way I would handle hit dice. I've seen some people say that they would either roll hit points for both classes and take the best, or average the hit points for both classes. This seems unnecessarily complicated to me. I would just use the hit dice of the best class, which would be Fighter for elves, most of the time. This would actually be a good idea for GMs who wish to eliminate level caps, since the downside to multi-class elves would now be that sometimes when they gain a level, they wouldn't gain a hit die, because they already have better hit dice from their other class.